A two-quadrant buck converter, also known as a synchronous buck converter, is a type of basic switch-mode de/de converter that is used to regulate voltage and provide efficient dc power transfer in energy systems. The traditional two-quadrant buck converter cell, shown in FIG. 20 (prior art), comprises a pair of complimentary power switches and input capacitor. An output L-C low-pass filter is employed when a small high frequency ripple for the output voltage is required. For steady state operation, switch S1 is turned “on” (i.e. switch S1 is closed) and S2 is turned “off” (i.e. switch S1 is opened) during time interval D·Ts. The converter duty cycle D, which ranges from 0 to 100%, represents the percentage time when switch S1 is on (and thus when S2 is off) during switching period Ts. During each time interval D·Ts, voltage vx at node x becomes equal to input voltage Vin as shown in FIG. 20. Voltage vx becomes zero when switch S1 is turned off (and thus when S2 is turned on) for the remainder of the switching period, due to the complimentary switching action of S1 and S2. Based on this discussion, the voltage vx can be viewed as having an average value of D·Vin with a set of high frequency switching harmonics. The L-C low-pass filter is designed such that it attenuates the high frequency switching harmonics of vx and allows the output voltage Vout to be equal to the average value D·Vin. Assuming the output voltage is externally regulated, the inductor current IL can be made to take on either a positive or negative average value through adjustment of the converter duty cycle, thus enabling bidirectional energy transfer between input and output terminals. Therefore, voltage regulation and bidirectional energy transfer can be achieved easily by suitable control of the duty cycle D in a two-quadrant buck converter.
It should be understood that a unidirectional variant of the bidirectional buck converter in FIG. 20 can be realized, by, for example, replacing switch S2 with a diode. The unidirectional buck converter can be employed for applications where only input to output power transfer capability is needed.
Multiple two-quadrant buck converter cells with associated filters can also be connected in series to form “classical cascaded buck converters”. FIG. 21 (i.e. prior art) shows a classical cascaded buck converter comprised of three individual dc/dc buck converter cells, each with associated output filtering, connected in series. The topology shown has three input ports and one output port. Each of the input ports and the output port consists of two terminals as shown. Observe each input port has an assigned reference terminal with its voltage defined relative to ground, i.e. vn1, vn2 and vn3. By chosen convention the reference terminals are selected such that they correspond to the bottom connection point of each input port capacitor. To limit voltages to ground, a single reference terminal is typically connected to ground. In FIG. 21, the reference terminal selected for ground connection is shown by the dotted connection from vn3 to ground. However, it must be stressed this choice is entirely arbitrary, i.e. any other reference terminal in FIG. 21 could have been connected to ground. The classical cascaded buck converter allows multiple input ports to exchange energy with a common output port, wherein the output voltage can be significantly higher than individual input voltages. This flexibility makes the classical cascaded buck converter suitable for a wide range of applications such as photovoltaic systems and battery management units.
Present state-of-the-art technology having similar application and functionality compared to the classical cascaded buck converter in FIG. 21 is the cascaded connection of two-quadrant buck converter cells that share a single L-C low-pass filter, shown in FIG. 22. However, with exception of the one ground-connected reference terminal, all other input reference terminal voltages for this topology, i.e. vn1 and vn2, are subject to undesired high frequency switching ripple voltage. As a result, energy sources that are connected to these input ports, for example, solar panels or batteries, will suffer from significant capacitive current to ground. In contrast, the classical cascaded buck converter with multiple low-pass L-C filters as shown in FIG. 21 can reduce the high frequency switching ripple voltage on vn1 and vn2, provided that individual L-C filter elements are sufficiently large. However, this comes at the expense of an overall increase in both the size and number of energy storage components (inductors and capacitors). The additional components increase the loss and cost of the classical cascaded buck converter.
It is, therefore, desirable to develop a new non-isolated dc/dc converter topology that can be designed to have an arbitrarily small high frequency switching voltage ripple magnitude at all input and output reference terminals, while achieving reduced component count, cost, and loss.